This invention relates to load pull testing of microwave power transistors using automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input and output of said transistors and analyze their large signal load-line behavior.
A popular method for testing and characterizing microwave transistors at high power nonlinear operation is “load pull” and “source pull” [10]. Load pull or source pull are measurement techniques employing microwave tuners and other microwave test equipment. The microwave tuners in particular are used in order to manipulate the microwave impedance conditions under which the Device under Test (DUT, or transistor) is tested (FIG. 1).
Whereas load and source pull provide information about the RF impedance presented to the DUT at the fundamental (Fo) and harmonic (2Fo, 3Fo, 4Fo, . . . ) frequencies, it does not yield any information about the actual voltage and current trajectory in real time. As long as the voltage and current peaks around the quiescent operating bias do not exceed breakdown values, the transistor operation is considered safe. (FIG. 2) This kind of operation is typically called “class A” or “class A-B”, or “class B”. However, in many cases the transistors are biased such as to allow current flow only when the signal exceeds a certain value. This is a “burst” kind of operation and allows a drastic increase in power added efficiency, since the current and power consumption is zero in the absence of an input signal (FIG. 3). Such operation conditions are usually called “class C”, “class F”, etc . . . ).
In a case like this the overall voltage excitation applied to the transistor between the control port (Gate) and the output port (Drain) can reach and exceed values into the avalanche breakdown range: VDG=VDS+|VGS|, (FIG. 3). In this case some transient avalanche current is going to flow at each peak voltage and the transistor will degrade rapidly towards material migration and electrical failure.
Further on the voltage-current trajectory, typically called the “load-line”, will depend on the complex impedance presented to the transistor (FIGS. 1-3). It is therefore important to investigate and associate said load-line not only to the signal amplitude applied to the DUT, but also to the complex RF load. Since the DUT is operating under strong nonlinear conditions and the input signal is distorted, there are harmonic components created, which need to be terminated with the corresponding harmonic loads. A setup that allows this test is a “harmonic load pull setup” as shown in FIGS. 1, 21.